1. Field of the Invention
The present invention relates to a frequency control loop circuit, and more particularly to a frequency control loop circuit which changes division ratios of a frequency synthesizer to oscillate frequencies in a broadband and acquire high precision.
2. Description of the Background Art
In general, RF communication systems use radio frequency (RF) signals of a high frequency band, and they employ a local oscillator (LO) to convert the RF signals of a high frequency band into baseband signals.
There is, however, a difference between a frequency generated from the LO or the RF frequency and a frequency desired by a system. The difference, or error, causes a time information error in the baseband.
In short, if a frequency of received signals is different from a clock sampling frequency or a frequency of a frequency oscillator in data communication or RF communication, it generates time information error and a receiving part cannot process the received signals exactly.
In order to solve such a problem, a receiving part compensates for the time information error by adding a frequency control loop to a frequency generator or a clock sample frequency generator in conventional receiver. The frequency control loop is called an automatic frequency control (AFC) loop. Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) has conventionally used to control frequency and perform compensation.
FIG. 1 shows a conventional frequency synthesizing device to describe a conventional automatic frequency control loop comprising a VCTCXO.
As shown in the drawing, the conventional frequency synthesizing device 100 comprises frequency synthesizers 110a to 110n, demodulators 120a to 120n and VCTCXO 130a to 130n. 
A frequency synthesizer 110a outputs an output frequency FOUT1 based on an input frequency FIN1 and a frequency of the VCTCXO 130a. 
A demodulator 120a detects an error in the output frequency FOUT1 outputted from the frequency synthesizer 110a and generates a control signal.
The VCTCXO 130a receives the control signal from the demodulator 120a, and feeds the oscillated frequency back to the frequency synthesizer 110a to thereby complete a frequency control loop.
Therefore a conventional automatic frequency control loop uses independent the VCTCXOs 130a to 130n and the frequency is controlled by adjusting a voltage of the VCTCXOs 130a to 130n based on the control signal.
The VCTCXOs 130a to 130n, however, have shortcomings that the production process is complicated and the production cost is high, compared to crystal or a clock generator.
When a single mobile communication terminal comprises a plurality of systems, for example, when the single mobile communication terminal comprises a Code Division Multiple Access (CDMA) system for mobile communication and a Digital Multimedia Broadcasting (DMB) system for a wireless broadcasting service, it should comprise the AFC loops independently from each other according to each receiving mode. Prior to the configuration of the AFC loops, a VCTCXO should be formed for each AFC loop.
In consequences, when the single mobile communication terminal adopts a plurality of systems, there are problems such as large mobile communication terminal, limited housing capacity, high production costs, and low productivity.